Electroless copper plating



United States Patent 3,326,700 ELECTROLESS COPPER PLATING Rudolph J. Zeblislry, 41 Glenwood Drive, Hauppauge, NY. 11787 No Drawing. Filed June 12, 1963, Ser. No. 287,220 15 Claims. (Cl. 106-1) The present invention relates to a new and improved process for the electroless plating of copper, and to a novel and improved electroless copper plating bath.

The present invention has for its object the provision of a novel and improved process for the electroless plating or deposition of a ductile, bright layer of copper on vari ous surfaces such as insulating members, metal, ceramic and other supporting surfaces. A further object of the invention is the provision of a novel and improved electroless plating bath which is simple to use, operates with certainty and which is economical in use. The invention provides an improved electroless copper depositing bath and process which produces a copper deposit of better color than has heretofore been realized, and which may exhibit substantially the same ductility as good electrodeposited copper.

The invention consists in the novel steps, processes, compositions and improvements herein shown and described.

Heretofore, a wide variety of electroless or autocatalytic copper plating baths and processes have been proposed and put into extensive commercial operation for the deposition of thin layers of copper upon insulating surfaces, ceramics, plastics and other materials. One of the important ingredients in such baths is a complexing agent for cupric ion.

Typical complexing agents for cupric ion in such baths are disclosed in US. Patent 2,996,408.

As noted in the aforementioned patent, however, the prior art has been unsuccessful in employing nitrolotriacetic acid and its salts as a cupric ion complexing agent 'in such autocatalytic copper deposition baths.

Heretofore, when an attempt has been made to use nitrilotriacetic acid and its salts, the resulting solution, although it started to plate, turned green and turbid and precipitated copper in a relatively short period of time, as noted in U.S. 2,996,408.

According to the present invention, it has been discovered that nitrilotriacetic acid and its salts may be used as a cupric ion complexing agent in the solution under discussion if combined with a cuprous ion complexing agent as taught herein.

The copper deposits produced by the solutions of the present invention are bright and ductile. Additionally, the baths of the present invention may be used until substantially exhausted, and may be replenished from time to time so that the remaining useful ingredients in the bath are more efficiently used, thereby reducing the overall cost of the operation.

In general, the electroless plating baths of the present invention are alkaline aqueous solutions comprising water soluble copper salts; such as, cupric sulfate, chloride, nitrate, and acetate; nitrilotriace-tic acid or salts thereof as a complexing agent for the cupric ions; an active reducing agent such ,as formaldehyde; and a complexing agent for the cuprous ions.

As the complexing agent for the cuprous ion may be used cyanide compounds, such as alkali metal cyanide, e .g., sodium cyanide, potassium cyanide, and the like, and organic ni-triles, such as chloroacetonitrile, alpha-hydroxynitriles, such as glycolnitrile and lactonitrile, and dinitriles, such as succinonitrile, iminodiacetonitrile, and 3,3-iminodiopropionitrile.

As noted above, the solutions described herein comprise, as a complexing agent for the cupric ions, nitrilotriacetic acid and salts thereof.

3 326,700 Patented June 20, 1967 ice Nitrilotriacetic acid corresponds to the structural formula N(CH COOH) Among the salts of nitril-otriacetic acid that may be used may be mentioned the alkali and alkaline earth metal salts, such as sodium, potassium, calcium and magnesium. Of the salts, the sodium and potassium salts are preferred.

The quantities of the various ingredients in the bath are subject to wide variation within certain ranges which may be defined as follows:

Copper salt 0.002 to 0.15 moles. Formaldehyde a- 0.03 to 1.3 moles. Nitrilotriacetic acid or salts 0.7 to 2.5 times the moles of copper. Cuprous complexing agent 0.00001 to 0.06 moles. Alkali metal hydroxide Suflicient to give a pH of 10 to 13. Water Sufficient to make 1 liter.

Sodium hydroxide and sodium cyanide are preferred over the corresponding more costly potassium and other alkali metal salts in the baths of this invention.

Cupric sulfate is the preferred copper salt but other soluble copper salts may be used, such as the nitrate, chloride, and acetate, as has already been indicated.

More preferably, the plating baths of the present invention are compounded within more narrow limits than set forth immediately above, and they consist essentially of:

A soluble cupric salt, preferably liter.

In considering these general formulae, and the specific working formulae which are set forth below, it should be understood that as the baths are used up in plating, the cupric salt, and the formaldehyde reducing agent may be replenished from time to time, and also that it may be advisable to monitor the pH and the cuprous complexing agent content of the bath, and to adjust it to its optimum value as the bath is used.

The amount of alkali metal hydroxide added should be such that the bath is maintained below pH 13. Preferably, the pH of the bath is maintained at about 12.

For best results, surfactants in an amount of less than about 5 grams/liter are added to the baths disclosed herein. Typical of such surfactants are organic phosphate esters, and oxyethylated sodium salts. Such surfactants are sold under the trade names Gafac RE 610 and Triton QS-lS, respectively.

The baths are ordinarily used at slightly elevated temperatures, such as from 25 to C., although they may be used at lower temperatures or at even higher tempera tures. As the temperature is increased, it is usual to find that the rate of plating is increased, and that the duetility of the deposit is increased to a slight extent, but the temperature is not highly critical and, within the usual operating range, excellent deposits are produced which exhibit greatly improved properties over those obtained with conventional baths and processes.

With electroless copper plating baths according to the present invention, the efiiciency of the copper recovery by electroless deposition from the bath often exceeds 90%, which is much greater than has heretofore been observed in working with conventional baths.

The cuprous complexer in the bath is an important feature and serves to prevent or minimize the formation of cuprous oxide in the bath and also appears to inhibit the formation of resultant hydrogen in the electroless deposited metal. Moreover, without the cuprous complexer, the bath has been found to be completely unsatisfactory as a stable plating solution.

The surface to be plated must be free of grease and other contaminating material. Where a non-metallic surface is to be plated, the surface areas to receive the deposit should first be treated, as in conventional processes, with conventional sensitizing and seeding solutions, such as stannous chloride (SNCI followed by treatment with a dilute solution of palladium chloride (PdCl Alternatively, extremely good sensitization is achieved by using a bath containing a mixture of stannous chloride and precious metal chloride, such as palladium chloride, the stannous chloride being in stoichiometric excess, based on the amount of precious metal chloride.

Where metal surface, such as stainless steel, is to be treated, it should be degreased, and then treated with acid, such as hydrochloric acid to free the surface of any oxide.

It will be understood that the foregoing general description and the following detailed description as well are exemplary and explanatory of the invention but are not restrictive thereof.

Example 1 A bath which is especially suited for the copper plating at about 25 to 30 C. of an epoxy or polyethylene terephthalate (Mylar) resin base sheet which is to form a printed circuit, and which has been treated with stannous chloride and'palladium chloride in selected areas corresponding to the circuitry to be produced, is immersed in a bath consisting of:

Cupric sulfate (CuSO 5H O) grams 5 Sodium salt of nitrilotriacetic acid (40% aqueous solution) ml 23 Sodium hydroxide grams 2 Sodium cyanide do 0.015 Formaldehyde (37%) ml Water, enough to make ml 1000 pH 11.5 to 12.0

At the end of about one day, the resin base sheet was coated with about a 0.5 mil thick, very bright, adherent copper plate on the selected areas corresponding to the circuits to be produced. A portion of the copper plate was stripped off the base sheet and found to have moderate ductility.

Results similar to those of Example 1 were obtained.

Example 3 As a comparison, Example 2 was repeated, with the exception that no sodium cyanide was added to the bath. The bath, although it started to plate, turned green and turbid and precipitated copper within a period of about five minutes. It was entirely unsatisfactory as a stable plating solution.

The fact that the bath could not be used without the presence of cyanide emphasizes the importance of the cuprous complexer in the baths of the present invention.

Example 4 Example 1 is repeated, with the exception that a steel base sheet, suitably cleaned, is substituted for the resin base sheet; similar results are obtained.

Example 5 Examples 1 and 2 were repeated with the exception that an equivalent amount of glyconitrile was substituted for sodium cyanide as the cuprous complexer. Similar results were obtained.

In the process according to the present invention, and as applied to the electroless deposition of copper on an insulating base such as sheet nylon, polystyrene, melamine resin, cast epoxy resin, and so forth, the surface of the sheet is first wetted with a detergent, such as a 1% water solution of Triton, or is roughened slightly by rubbing with an abrasive. The surface is then thoroughly cleaned with an alkaline cleaner, such as a hot solution of trisodium phosphate in water, rinsed with water and is sensitized with a solution containing about 70 grams of stannous chloride (SnCl 2H O) and 40 ml. of 37% hydrochloric acid per liter for a period of about 10 minutes. The surfaces are again thoroughly rinsed with water, and may be further treated with a solution of from 0.5 to 25 ml. per liter of hydrochloric acid (37% The surface is then seeded by treating the surface with a solution of 0.10 grams of palladium chloride (PdCl 1 ml. of 37% hydrochloric acid and 1 ml. of Triton Xl00 per liter, for 2 or more minutes. Thereafter, the surface is again thoroughly rinsed with water and is then ready for immersion in the electroless depositing bath, such as the bath of Example 1, or Example 2.

Alternatively, with base materials of the type described, sensitization may be achieved by use of the stannous chloride-precious metal chloride sensitization solutions containing about 1 g./l. palladium chloride, 20 g./l. stannous chloride, and 200 ml./l. of 37% hydrochloric acid.

A similar sensitizing and seeding treatment may be used as a preliminary treatment in connection with many other base materials, although when a metal surface is to be plated it is seldom necessary to do more than treat the metal surface, as by a solvent or a hot solution of an alkaline cleaner (e.g., trisodium phosphate), to insure that it is grease-free, and with an acid, such as hydrochloric acid if the surface has any oxide coating thereon.

The present invention is not limited to the use of the foregoing specific sensitizing and seeder solutions, and in many instances it is only necessary to roughen the surface of the insulating base material, or to thoroughly clean the metal surface.

The invention in its broader aspects is not limited to the specific steps, processes and compositions shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantage.

What is claimed:

1. An alkaline electroless copper plating bath which comprises water, -a water soluble copper salt, a complexing agent for cupric ion which is a member selected from the group consisting of nitrilotriacetic acid, alkali metal salts of nitrilotriacetic acid and mixtures thereof, formaldehyde, and a small effective stabilizing amount of a complexing agent for cuprous ion which is a member selected from the group consisting of metal cyanide compounds and organic nitrile compounds in an amount of 0.000 to 0.01 mole per liter.

2. The electroless plating bath of claim 1 wherein said cuprous ion complexing agent comprises potassium cyanide.

3. The electroless plating bath of claim 1 wherein said cuprous ion complexing agent comprises sodium cyanide.

4. The electroless plating bath of claim 1 wherein said cuprous ion complexing agent comprises an organic nitrile compound which is a member selected from the group consisting of chloroacetonitrile, glycolnitrile, dinitriles, succinonitrile, iminodiacetonitrile and 3,3-iminodipropionitrile.

5. A process of electroless copper plating which comprises immersing the receptive surface to be plated in an alkaline bath comprising water, a water soluble copper salt, a complexing agent for cupric ion which is a member selected from the group consisting of nitrilotriacetic acid, alkali metal salts of nitrilotriacetic acid and mixtures thereof, small effective stabilizing amount of a complexing agent for cuprous ion which is a member selected from the group consisting of alkali metal cyanide compounds and organic nitrile compounds in an amount of 0.00001 to 0.06 mole per liter.

6. The process of claim 5 wherein said cuprous ion complexing agent comprises potassium cyanide.

7. The process of claim 5 wherein said cuprous ion complexing agent comprises sodium cyanide.

8. The process of claim 5 wherein said cuprous ion complexing agent comprises glycolnitrile.

9. The bath of claim 1 wherein the pH is adjusted to between about to 13 and wherein said cuprous ion complexing agent is glycolnitrile.

10. A process of electroless copper plating which comprises immersing the receptive surface to be plated in an alkaline bath comprising: water; a water soluble copper salt; a complexing agent for cupric ion which is a member selected from the group consisting of nitroltriacetic acid, alkali metal salts of nitrilotriacetic acid, and mixtures thereof, and a small effective stabilizing amount of a euprous ion complexing agent which is a member selected from the group consisting of metal cyanide compounds and organic nitrile compounds in an amount of 0.0001 to 0.06 mole per liter.

11. A process as claimed in claim 10 in which the pH is adjusted between 10 to 13 and wherein said cuprous ion complexing agent is an alkali metal cyanide compound.

12. A process of electroless copper plating which comprises immersing the receptive surface to be plated in a bath comprising water; a water soluble copper salt, from 0.002 to 0.15 mole per liter; alkali metal hydroxide,

enough to give a pH of about 10 to 13; a complexing agent for cupric ion which is a member selected from the 5 group consisting of nitrilotriacetic acid, sodium salts of nitrilotriacetic acid and potassium salts of nitrilotriacetic acid and mixtures thereof, a small effective stabilizing amount of a cuprous ion complexing agent which is a member selected from the group consisting of metal cyanide compounds and organic nitrile compounds in an amount of 0.00001 to 0.06 mole per liter; and formaldehyde, 0.03 to 1.3 moles per liter.

13. The process of claim 12 wherein the pH is maintained at about 12.0 throughout the plating operation and wherein said cuprous ion complexing agent is a member selected from the group consisting of chloroace-tonitrile, glycolnitrile, dinitriles, succinonitrile, iminodiacetonitrile and 3,3'-iminodipropionitrile.

14. An alkaline bath for the electroless plating of copper which comprises water, a complexing agent for cupric ion which is a member selected from the group consisting of nitrilotriacetic acid alkali metal salts of nitrilotriacetic acid and alkaline earth metal salts of nitrilotriacetic acid and mixtures thereof, formaldehyde, and small effective stabilizing amount of a cuprous ion complexing agent which is a member selected from the pump consisting of metal cyanide compounds and organic nitrile compounds in an amount of 0.00001 to 0.06 mole per liter.

30 15. The bath of claim 1 wherein the complexing agent for cuprous ion is a member selected from the group consisting of alkali metal cyanide compounds.

References Cited UNITED STATES PATENTS ALEXANDER H. BRODMERKEL, Primary Examiner. MORRIS LIEBMAN, Examiner.

I. E. CARSON, D. J. ARNOLD, Assistant Examiners. 

1. AN ALKALINE ELECTROLESS COPPER PLATING BATH WHICH COMPRISES WATER, A WATER SOLUBLE COPPER SALT, A COMPLEXING AGENT FOR CUPRIC ION WHICH IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF NITRILOTRIACETIC ACID, ALKALI METAL SALTS OF NITRILOTRIACETIC ACID AND MIXTURES THEREOF, FORMALDEHYDE, AND A SMALL EFFECTIVE STABILIZING AMOUNT OF A COMPLEXING AGENT FOR CUPROUS ION WHICH IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF METAL CYANIDE COMPOUNDS AND ORGANIC NITRILE COMPOUNDS IN AN AMOUNT OF 0.00005 TO 0.01 MOLE PER LITER. 